Unless you've grown big (the AI gangs up on the top three) the AI largely bases its decision on the total military strength in the border provinces. This actually means that it is a disadvantage to have a short border in terms of number of provinces, since you logically build up a "border garrison" to a size proportional to the number of border provinces. What the AI doesnt do is look at the garrison inside cities, or whether forces can reach the hot spots within a turn or not. It only looks at the strength in the entire province (I suspect this is an AI remnant from the earlier Total War games that had a risk style map).

So what can be done about it?

1) Avoid having a lonely province surrounded by the territories of one adversary (Venice, Sofia, Antwerp, Caen, etc) unless you have the money to pay for upkeep of a few extra stacks sitting in forts, which gets expensive fast.

2) Let the AI hold lonely provinces surrounded by yours. As France I take Brussels and Rennes, and the English have NEVER started a war since they compare their Caen force strength to that of my four border provinces and find themselves outnumbered even if I only keep militia stacks in the cities. I also let the Danes take Antwerp, and if I garrison Brussels properly they wont attack me due to being outnumbered.

3) Avoid certain islands unless you control all nearby mainland provinces. Corsica and Sardinia (Ajaccio-Cagliari) will be the targets of all manners of weird attacks, as the AI makes even less rational decisions when attacking islands.

4) If you have a 1:1 province ratio along the border with an AI faction, build one fort per province and put in extra stacks there. I manage to keep the notorious Milanese perfectly peaceful if I have full garrisons in Marseille and Dijon and have one fort per province (preferrably in a strategic mountain pass and within walking distance from the city).

5) As a catholic: focus on excommunicated factions (call crusades against them if feasible) when fighting catholics, and bring siege machinery when attacking non-excommunicated ones. Make an alliance with the pope in the very beginning (and dont go rampant on sacking prior to that, as that will make the alliance harder to come by).

6) Release prisoners after battle. This helps your reputation and makes it a bit easier to make deals with the AI. I find that if I keep doing this I can afford to sack cities and still keep a reliable reputation:p

7) The AI is very unlikely to make peace with you until you either outnumber him in the border provinces or dont have any common border at all.

Basically it is defensively sound to make the AI have a short border with you rather than the other way around, which is the total opposite of strategic good sense:p